The invention relates to melting of metals and, more particularly, to a rotary furnace process and apparatus applicable to continuous melting of predominantly metallic charge materials.
Known commercial melting processes have inherent processing difficulties and disadvantages only partly overcome by improvement to design and operating practice. As a ferrous melting example, in electric-arc furnace (EAF) melting of iron and steel scrap, unmelted charge materials are heated to melting temperature with solid surfaces contacting ambient air or hot oxidizing gases, thereby generating oxide particulates and lowering yield. The heat input is focused on a small area within the furnace relative to the total area occupied by the charge materials. Furthermore, carbon monoxide generated by oxygen injection into the metal bath is only partially burned to carbon dioxide by post-combustion before exit from the EAF, and only a fraction of the heat so released is transferred back into the charge. Cupola melting has like disadvantages, as well as limitation to production of cast iron, rather than steel. As a nonferrous example, reverberatory aluminum melting furnaces are widely applied commercially and focus the location of unmelted charge in a small area in relation to the sources and broad distribution of available heat in the furnace.
Elongated rotary melting furnaces employing a partially melted bath into which a solid charge is fed overcome some of the above deficiencies by means of continuous bath stirring and advancing action, in combination with efficient flame-to-wall, followed by wall-to-charge heat transfer during each furnace rotation. Access for introduction of the metallic charge materials, fluxes and reagents into the process, however, is only via annular furnace end openings, whereas the process mass transfer, heat transfer and process chemical reaction requirements vary and are distributed along the length of the reaction zones. As an example, when cold charge materials are introduced into a partially melted metal bath only adjacent to the entry opening, unmelted material may aggregate, creating islands of partially melted material and the like when, at the same time, charge further along the furnace is fully melted and becoming overheated. Unmelted islands of metal exposed to hot furnace gases are also subject to increased oxidation and loss as oxide particulates. Such problems obviously represent deficiencies in the control of process chemical reactions, mass transfer and heat transfer, and can also be a restriction on the maximum charging and production rates obtainable. It is therefore a principal object of the invention to distribute the melting heat requirement of unmelted charge materials along the elongated reaction zones according to the distribution of heat available to effect melting, with the corollary object of fast melting of the metallic charge materials.
Metallic charge materials characteristically carry varying percentages of metal oxides and other impurities as metal oxides, other metals, other compounds, dissolved gases, other elements such as phosphorous, sulphur and the like. Fluxes and additive reagents are required as components of the charge materials for reaction with these impurities, along with the metallic charge during processing, to obtain effective process parameters and desired end product composition following melting. Perhaps the most common example of an additive reagent is carbon for reduction of metal oxides to increase the yield of metal and/or for alloying the metal to obtain a specific range of dissolved carbon in the melt. It is naturally desirable that the carbon be introduced at the most effective locations to obtain the desired process reactions, such as reaction with metal oxides or oxygen, evolving carbon monoxide (CO) into the furnace gases, and then effecting a high degree of CO post-combustion (PCD), with a good heat transfer efficiency (HTE) into the furnace charge of the heat so liberated prior to the furnace gases exiting the furnace, and for control of the product composition. It is thus another principal object of the invention to distribute the introduction of fluxes and reagents along elongated process reaction zones according to the distribution of process chemical reaction requirements.
The invention provides a process and apparatus for continuous metal melting in a horizontally-disposed elongate rotary furnace comprising maintaining a partially melted bath of metal carrying a floating layer of slag in an elongate gas-solid-liquid reaction zone heated by a hot gas stream passing over the metal and slag within the furnace; conveying solid charge materials comprising metallic materials, fluxes and additive reagents through an annular furnace end opening and along the gas-solid-liquid reaction zone and downwardly projecting them into the bath; traversing the position of said downwardly projecting successively backwards and forwards thereby distributing the entry location of charge materials into the bath along a longitudinal traverse span, and allowing liquid metal to flow out of the gas-solid-liquid reaction zone thereby providing for replenishing the bath with fresh solid materials. Said traverse span preferably comprises a major portion of the length of the gas-solid-liquid reaction zone.
When applied to granular or pelletized charge materials less than about 3 cm. in size, for example, DRI pellets, granular iron carbide, pulverized coal, lime, crushed and screen limestone and ferroalloy additives, said conveying suitably comprises entraining the charge materials and propelling them by pressurized carrier gases through a solids injection lance cantilevered longitudinally within the hot gas stream in the gas-solid-liquid reaction zone and said downwardly projecting comprises issuing a jet of charge materials and carrier gases downwards from a lance nozzle into the partially melted metal bath whilst stroking the lance successively backwards and forwards distributing the entry location of charge material longitudinally along said traverse span. When applied to larger-sized charge materials, such as recycled scrap metals, pig iron, hot briquetted iron (HBI), lump coal or coke, lump fluxes and the like, said conveying suitably comprises propelling the charge materials by oscillation of an oscillating conveyor, also cantilevered along the gas-solid-liquid reaction zone, and said downward projecting comprises dropping the charge materials downwards from a discharge lip of the conveyor into the bath whilst stroking the conveyor backwards and forwards. Process requirements usually favor charging by a combination of oscillating conveyor and solids injection lance, in which case some overlapping of the lance and conveyor traverse spans is usually desirable, in which case the invention includes controlling the travel cycle time intervals and relative positions of the lance nozzle and conveyor discharge lip to avoid interference during entry between charge materials issuing from the lance nozzle and those dropping from the conveyor discharge lip at any time of passage across the common span of travel.
Further, the rate of charge material flow can also be varied at different positions along the traverse span, even including interruptions, in order to realize longitudinal distribution of charge material entry according to desired process parameters. This can be effected either by varying the velocity of stroking or, in the case of lancing, varying the lance inlet feed rate.
Metals usually carry surface oxides, for example, iron rust, DRI or other pre-reduced virgin materials may also contain substantial content of unreduced residual metal oxides. Metals are also subject to high-temperature oxidation in-process. Also, dissolved carbon is often desired as a product constituent, such as with iron and steel melting. The charge materials therefore typically include carbonaceous materials carrying carbon as an additive reagent for reduction of the oxides within the metal and slag bath, releasing carbon monoxide (CO) into the hot gas stream, which represents an unburned combustible or fuel. Selectively injecting oxygen into the hot gas stream facilitates the post-combustion of most of this CO within the process elongate reaction zones, and also recovery of the heat so released by direct in-process heat transfer back into the partially melted bath, with a PCD and HTE higher than that attainable by prior art processes.
The process and apparatus of the invention is most suitably applied with the rotary furnace length further elongated to incorporate a gas-liquid reaction zone adjoining the gas-solid-liquid reaction zone into which the liquid metal flows and accumulates for refining reactions and temperature adjustment prior to discharging from the furnace. This zone is heated by a burner from which the products of combustion form the hot gas stream flowing on into the gas-solid-liquid reaction zone countercurrent to the general movement of materials and exhausting through the annular end opening adjacent to the gas-solid-liquid reaction zone. The liquid metal may be discharged by periodically stopping the furnace rotation and opening a tap-hole discharging into a ladle or, alternatively, siphoning the metal continuously or semi-continuously via a refractory tube inserted into the metal through the furnace annular end opening entering an adjacent vacuum vessel external to the furnace, from which the metal is withdrawn for casting or further processing. Slag may be discharged by overflowing the lip of an annular end opening, including skimming as required or optionally assisted by end-wise furnace tilting through a small angle or, alternatively by a vacuum slag removal system such as described in my U.S. Pat. No. 5,305,990.
The process and apparatus is applicable to melting of various metals, for example, ferrous metals comprising iron and steel scrap, pig iron, DRI pellets, HBI, and also various virgin or recycled forms of non-ferrous metals such as copper, aluminum, lead, zinc, chromium, nickel, tin and manganese. Mixtures of metals and metal oxides can be processed and it is adaptable to acidic or basic slag and refractory practice. It accommodates a wide range of charge material sizes, ranging from fine granular particles charge by pneumatic injection up to conveyor-sized pieces of recycled scrap metals. It facilitates continuous melting whilst retaining the options of discharging product either continuously, intermittently or batch-wise. It also facilitates high heat transfer rates throughout the process reaction zones and avoids localized overheating or undercooling, as well as provides good metal-slag interaction towards composition approaching chemical equilibrium to realize high product yields and consistent chemical composition. The invention therefore represents a fast, clean, quiet, thermally efficient and versatile technology for metal melting requirements.